In general, fuel cells are power generation systems that directly convert the chemical reaction energy of hydrogen and oxygen contained in a hydrocarbon-based substance, such as methanol, ethanol, and a natural gas, into electrical energy. Such a fuel cell generally includes an anode electrode to which a fuel is supplied, a cathode electrode to which an oxidizer is supplied, and a cation exchange membrane positioned between the anode electrode and the cathode electrode. In the fuel cell, the oxidizer generally uses oxygen or air.
Much attention has been paid to the fuel cells as systems for clean energy conversion since the fuel cells are highly efficient and discharge/emit a small amount of pollutants. However, the fuel cells have yet to be successfully deployed on a commercial scale since they are expensive and have low technical reliability. In particular, an oxygen-reduction reaction taking place in the cathode electrode (‘air electrode’ or ‘reduction electrode’) is about 106 times later than a hydrogen-oxidization reaction taking place in an anode electrode (‘fuel electrode’ or ‘oxidation electrode’). Thus, the oxygen-reduction reaction taking place in the cathode electrode is not only a main factor that determines a total reaction speed but also a main cause that determines the performance of a fuel cell.
To increase the speed of the oxygen-reduction reaction, a platinum catalyst is used in cathode electrodes of most of fuel cells at present. The platinum catalyst has high electrical conductivity and good catalyst characteristics but is expensive and has a limit to increasing a surface area in which a catalytic action takes place. To save manufacturing costs, the platinum content of the cathode electrode may be reduced or a non-noble metal catalyst has been requested to be developed as an alternative catalyst.
To this end, redox fuel cells using various redox couples having high speeds of reaction have been developed. In the redox fuel cells, since a redox couple that is regeneratable is used instead of an anode electrode and a cathode electrode, a kinetic loss is reduced and a noble metal catalyst may not be used. However, a complicated regeneration process, e.g., a microbial process, should be performed to increase the speed of regeneration at present.